Pressure sensitive touch pads are commonly used with personal digital assistants (PDAs) and other portable electronic devices. Pressure sensitive touch pads convert mechanical pressure, typically applied by a stylus, into an electrical output. The stylus is typically used to enter characters or make menu selections. Many conventional touch pads do not include a display that traces (i.e., shows) the path of the stylus. As a result, it can be difficult for users to accurately use a stylus to input character data. This is particularly a problem with complex characters, such as characters that are a part of oriental languages. As the accuracy of the input data decreases, so does the accuracy of the output electrical data.
One approach to displaying the path traced by a stylus includes the use of an active stylus. An active stylus generates an electric or magnetic field and changes the state of a display placed over a touch pad. One of the disadvantages of using an active stylus is that an active stylus cannot be interchanged with other styluses. Another disadvantage is that active styluses that include electrical components are expensive, when compared with passive styluses.
Electrically addressable displays have also been placed over touch pads to display the path traced by a stylus. Information detected by the touch pad is transmitted to a display driver circuit. The display driver circuit energizes patterned display electrodes and causes the display to display the path traced by the stylus. Display driver circuits increase the size and expense of the device. Another drawback to devices that include electrically addressable displays placed over touch pads is that such devices require frequent and inconvenient calibration steps in order to keep the trace generated by the display aligned with the actual position of the stylus.
Therefore, there exists a need in the art for a pressure sensitive input device that displays the trace of a stylus without the use of an active stylus or complex driver circuits.